Motion is everywhere in media. For instance, when you press "play" in a video player you start moving through the film. Pressing "next" in a slide show moves you to the next slide. These are motions through the presentations.
Shared motion is putting the motion online, making it possible to share media motions between multiple devices, people or both. Basically, the play button is moved into the Internet. This makes all devices act as terminals, or windows, into a media experience. These windows can be shared, they can display the same content or different aspects of a shared storyline. They can render the content to exploit the possibilities of each device, they can join or leave when they want and they can collaborate in controlling the presentation.